With a desire to improve steam turbine efficiency there is an interest to increase steam turbine operation temperature. The higher temperatures can create new material challenges. For example, a known heat resistant 12-Cr steel alloy, which can be suitable for use up to about 610 degrees Celsius, is not suitable when temperatures exceed about 650 degrees Celsius. In such cases a more suitable, but more expensive, nickel alloy can be desired.
Special material considerations are used not only for valve bodies but also for valve actuators. However, due to internal works of actuators, problems caused by high temperature may not be solved merely by material selection. The temperature exposure of the actuator can also be limited.
One way of achieving this is to cool the valve casing so that excessive heat is not transferred to the actuator. U.S. Pat. No. 7,481,058 B2 describes one such cooling configuration in which the valve casing is provided with cooling ports and means to pass cooling medium through these ports. Such configurations can be complex and further involve the configuration and supply of a cooling medium.
An alternative is to provide a distance spacer, made of a high hot strength material, between the valve casing and the actuator. Having a high hot strength material means the connector spacer can tolerate the conducted heat energy from the valve casing while its length further enables it to lose some of this heat convectively. This can create a temperature gradient between the casing and the actuator. By configuring the spacer length, a significantly large temperature gradient can be created so that the actuator is not exposed to excessive temperatures. However, in providing the temperature gradient, the space should be configured to withstand the high temperature of the valve casing. For valves in high steam temperature service this can mean the spacer should be made of expensive nickel-alloy.
Exemplary embodiments as disclosed herein can provide alternate simple cost effective solutions that overcome the excessive heat exposure of steam valve actuators when placed in, for example, about 650 degrees Celsius steam service.